superconducting rf materials university collaboration status lance cooley june 07, 2010
TRANSCRIPT
Superconducting RF Materials University Collaboration
STATUS
Lance Cooley
June 07, 2010
Status of the Collaboration
June 09, 2010
ILC ART Review at Fermilab 2
• FNAL, UChicago, IIT, Northwestern, Florida State– UIC not directly funded but is
involved– IIT work occurs partly at ANL
• FNAL – Cooley– Peoples Fellow A. Romanenko
(SRF surface science)– NW Ph.D. student Denise Ford
(computational chemistry)• UChicago – Sibener
– M. Nakayama, T. Wang (oxidation fundamentals, STM)
• IIT – J. Zasadzinski– T. Proslier (ANL) & TBD (PCT,
ESR)
• $1.5M / 3 years, started 9/1/09– POs issued to collaborators on
separate schedules
• Northwestern – D. Seidman– McCormick Appointment – Yoon-
Jun Kim
(LEAP microscopy of Cottrell atmospheres)
• Florida State – P. Lee– Z. Sung (FIB isolation and
electron microscopy)• UIC – R. Klie
– Atomic resolution EELS
Recent achievements
Subtle effects of contaminants• Oxidation
– Thick, stressed oxidation (stuck HPR) initiates quench (JLab result)
– A grain boundary in the hot spot is also a source of magnetic scattering (IIT – at right)
– Other locations seem benign – the oxide itself is not the origin of quench. Defects in the oxide continue to be plausible sources
• ESR data, too – Nb 4+ (IIT)
• Implications– Avoid stress to the natural oxide– Avoid “flash” oxidation
• Acid trapped in pits hit with rinse water– Hunt for the oxide defects
June 09, 2010
ILC ART Review at Fermilab 3
Point-contact tunneling:Normal electrons at
oxide hot spot
Ohm’s law
In hot spot
Typical superconductor response at cold areas
(red, green)
2D
Ciovati – Jlab, Proslier – ANL, Zasadsinzki – IIT,Cooley, Romanenko – FNAL
Recent achievements
Subtle effects of contaminants• Hydrogen
– During EP, sulfate anodization (oxidation) of niobium may produce threading dislocations (A.Romanenko)
– EP also loads metal with hydrogen (unlike BCP)– Hydrogen binds to Nb vacancies, prevents
removal of threading dislocations (so-called Cottrell atmosphere)
– Mild baking (120°C) releases vacancies and thereby restores dislocation climb, which improves surface resistance
• Implications– 800 °C bake to remove surface hydrogen– Final EP must be cold– Re-tool surface science to find H (e.g. LEAP)– Positron annihilation, µ-SR
June 09, 2010
ILC ART Review at Fermilab 4
Plot: Visentin – SRF09
Cooley, Romanenko – FNAL Seidman - NWU
H
Vac.
Baked – mobile H
No bake – bound H
Materials achievements – past 12 months
Understanding electrochemistry• Do things go wrong?
– Coupon EP is glossy, with no grain boundary contrast– Cavity EP is less glossy, with visible grain boundaries– Agitation and stirring circulate fluorine to coupon
surface, producing grain-boundary contrast• High temperature reduces viscosity, promotes circulation
(H. Tian Ph.D. thesis)• Some grains may etch quickly, leaving faceted pits
• Implications– Keep EP cold and don’t agitate
• Do not use acid flow as the coolant! Instead, apply external cold water spray to EP tool, and turn back flow.
– Final EP will then be slow – use alternate process (tumbling) to make up processing time
• Process MUST pre-condition surface to a roughness comparable with the thickness of fluorine diffusion layer (~40 µm at 20 °C – Reece et al.)
June 09, 2010
ILC ART Review at Fermilab 5
Non-typical EP coupon showing grains (FNAL)
Summary – Good materials science AND implications for SRF
• Collaboration PIs are advancing basic materials topics– Cottrell atmospheres– Spin-resonance and positron annihilation spectroscopy– Oxidation kinetics
• Collaboration work is stimulating new tool development– Low-T STM at UChicago– Electron di-chroism in the STEM at UIC– LSCM and EBSD at FNAL
• Implications of work have direct impact on SRF process– Avoid excessive oxidation, “flash” oxides, and mechanical damage to
oxide layer– Take better care of hydrogen– Don’t heat or agitate the electrolyte if it can be helped – not only does
this make surfaces rougher, it also promotes the subtle complications of oxide and hydride chemistries
– New question: Is contamination more important than roughness?
June 09, 2010
ILC ART Review at Fermilab 6